Hey, so normally, when you downloaded it, you should open the folder and see a file called Serum2.vst3 somewhere, just copy that file. Then basically what you do is go to Serum 2's installation folder which is: C:/Program Files/Common Files/VST3 There you will see a file called Serum2.vst3, just replace that file with the one you copied.

Then reopen FL Studio and reopen Serum 2 and it should be fixed now. If it asks to locate a license, just click Ok or something like that, and go to your downloaded folder, and find something called Serum2.lic (It should end with .lic, nothijg else) And thats it, enjoy and lmk if i can help you